Metro Charlotte has for the first time dropped off the American Lung Association’s annual list of the smoggiest U.S. cities.

Charlotte’s been a regular on the list of ozone-polluted metro areas since the list began in 2000. The city has struggled for decades to meet federal ozone standards, largely because of tailpipe emissions.

Ozone, sometimes called smog, poses a public health problem. The invisible gas forms in hot, sunny weather and can trigger asthma attacks and worsen respiratory conditions.

But the rankings released Wednesday don’t include Charlotte among the 25 cities with the worst ozone pollution.

Metro Charlotte had improved in the ratings in recent years, dropping from eighth-worst in 2009 to 19th in 2013. The rankings are based on data collected over three-year periods.

Don’t make too much of Charlotte’s absence this year, the association warns. Ozone worsened over much of the country because of unusually hot weather in two recent summers. Charlotte benefited by comparison.

The association says 122,000 Mecklenburg County residents have lung diseases. An additional 242,000 people are under 18 and 91,000 over 65, and both groups are vulnerable to air pollution.

This week’s rankings don’t include a record low year for ozone in 2013. The Charlotte region never broke the federal standard that summer, when frequent rain inhibited the conditions that breed ozone.

But cleaner vehicle standards and a state crackdown on emissions from coal-fired power plants, which form ozone, also contributed. Previous years with weather similar to last year’s tallied more smoggy days.

The overall trend is toward lower emissions and ozone levels, she said.

The Charlotte region still hasn’t met a newer, tougher ozone standard set in 2008. The deadline to meet that federal standard is the end of 2015. Rhodes said it might be attained sooner if conditions remain good this summer.

The Charlotte Regional Transportation Planning Organization certified April 16 that future road-building projects using federal money won’t make air pollution worse. The determinations are required every four years.